The present invention relates to single phase silicon aluminum oxynitride-boron nitride composites and articles and to a method of making the composites and articles, and more particularly, to a multiphase ceramic composite consisting of boron nitride dispersed throughout a matrix of single phase silicon aluminum oxynitride, a window transparent to electromagnetic radiation made therefrom and to a method for making the same.
High-speed vehicles, such as high-speed aircraft, may be required to pass through a hostile environment. Under these conditions, it may become necessary to provide protection for certain communication and detection apparatus carried by such a vehicle from sources of heat shock and pressure shock. In such high-speed vehicles, such shock is typically due to frictional contact with the ambient environment. Protection from heat and pressure shock must be provided without detracting from the ability of the protected apparatus to communicate with the outside of the vehicle by means of electromagnetic radiation within the frequency spectrum of interest. A common way of providing the desired protection is to place the apparatus behind a suitable window, which is transparent to the desired frequency spectrum of electromagnetic radiation, sometimes referred to as an electromagnetic window or an antenna window.
Currently available compositions for providing such an electromagnetic window or antenna window often lack the requisite mechanical strength, toughness and thermal shock resistance. In U.S. Pat. No. 4,666,873, Morris, Jr. and Tanzilli have provided an improved window transparent to electromagnetic radiation in a predetermined frequency spectrum wherein the window is made from a ceramic material containing about 0.01 percent to about 35 percent by volume boron nitride and aluminum nitride. In U.S. Pat. No. 4,666,873, which is incorporated herein by reference in its entirety, an article is fabricated by mixing powders of the specified materials; shaping the mixture into the desired form of the article; and densifying the mixture in a non-oxidizing medium at a temperature, pressure and time sufficient to form a structural ceramic. Morris, Jr., and Tanzilli found that the resulting ceramic article could be used to protect apparatus from damage due to heat or pressure shock without detracting from the ability of the apparatus to communicate through the ceramic article (window) by means of electromagnetic radiation in a predetermined frequency spectrum.
Although U.S. Pat. No. 4,666,873 provides a ceramic article, such as an electromagnetic window, having excellent characteristics, it is always desirable to provide alternative composites and articles made therefrom wherein one or more of the properties are improved. A composite for use as an electromagnetic window ideally possesses transparency to electromagnetic radiation in a predetermined frequency spectrum over a wide temperature range, congruent vaporization kinetics, appropriate dielectric properties, thermal shock resistance over a wide range of heating rates, mechanical strength and toughness, spall resistance, high refractoriness, and a low erosion rate and ablation rate.
In a copending patent application Ser. No. 07/287,233 of Dodds and Tanzilli filed concurrently herewith on Dec. 21, 1988 for Docket No. 39-RE-2767 entitled Silica, Boron Nitride, Aluminum Nitride, Alumina Composite, Article and Method of Making Same and assigned to the instant assignee, it was discovered that several of the foregoing properties were improved in a multiphase ceramic composition containing about 47% to about 52% by weight aluminum nitride, about 23% to about 27% by weight silica, about 3% to about 7% percent by weight alumina and the balance boron nitride, the boron nitride being one phase in the composite and the reaction products of aluminum nitride, silica and alumina being the matrix phase in the composite, wherein the boron nitride phase is uniformly distributed in the form of discrete particles throughout the multiphase reaction products of aluminum nitride, silica and alumina. It was found that the multiphase ceramic composite of silica, boron nitride, aluminum nitride and alumina resulted in a composite combining moderate thermal expansion, low thermal conductivity, improved high temperature radar transmission and improved toughness to make an excellent antenna window material. However, it remains desirable to provide composites, and articles made therefrom, having one or more of the foregoing properties improved.
Single phase silicon aluminum oxynitrides corresponding to the formula Si.sub.6-z Al.sub.z N.sub.8-z O.sub.z where z is greater than 0 and less than or equal to 5 have been prepared using magnesium oxide in an amount of not more than 5% by weight to form a magnesium silicate glass to aid in densification of the ceramic material formed therefrom in U.S. Pat. No. 3,991,148 which is incorporated herein in its entirety for its teachings relating to single phase silicon aluminum oxynitride and the formation of the single phase silicon aluminum oxynitride from effective amounts of silica, alumina and aluminum nitride. Ceramic materials are also made from single phase silicon aluminum oxynitride in U.S. Pat. No. 4,113,503 wherein the silicon aluminum oxynitride is derived from various compounds containing two or more of the elements silicon, aluminum, oxygen and nitrogen. U.S. Pat. No. 4,113,503 is incorporated by reference herein in its entirety for its teachings relating to the method of forming a single phase silicon aluminum oxynitride. Although U.S. Pat. Nos. 3,991,148 and 4,113,503 teach single phase silicon aluminum oxynitrides and ceramic materials derived therefrom, it is desirable to improve various properties thereof including toughness so that articles formed therefrom can withstand heat shock and pressure shock in hostile environments.